In recent years, ink jet printers have been gaining acceptance as a means for reproducing an image which may include a picture as well as alphanumeric information. In such systems, one or more fine jets of ink are controlled by an electrical input. The controlled jets of ink are directed to a receiving medium such as paper on which the image is to be reproduced.
The control signals for the ink jets are obtained by optoelectric scanners which allow the conversion of a picture on a master into electrical analog signals, the magnitudes of which are proportional to the gray level of the picture elements, or pixels, in the picture. To this end, the picture is scanned linewise by an optoelectric device on a scanning head in raster fashion as with a television image.
The simplist type of scanner is the drum scanner in which the picture to be reproduced is mounted on a drum which is rotated at a high speed. A scanning head passes along the surface of the drum parallel to the drum axis at a slower speed. If a color image is to be reproduced in color, a color separation must be included in the scanning head to produce at least three analog signals, each corresponding to a specific color level of the particular pixel being observed at each instant. Typically, magenta, yellow and cyan color levels are obtained, and preferably a white level is also obtained.
To provide the separate color signals for each pixel of the image, the pixel is usually imaged onto a prism by a lens. The three color prism separates the original multicolor pixel image onto three separate pictures, each of which consists of one of the primary colors in the original picture. Such a method is commonly used in television cameras. A primary disadvantage to such an approach is that three color prisms which can properly separate the single pixel are very expensive.
The use of a light guide having three legs to direct light of different colors from a master to distinct photoelectric detectors has been suggested in U.S. Pat. No. 2,196,166 to J. W. Bryce. However, that patent was not for a facsimile system. The Bryce patent was concerned with detecting color coded spots, each spot comprising only one color to be detected. In that system, it was not necessary to Bryce to be concerned with obtaining precise relative intensity levels of separate colors from each single pixel. Further, the coded spots were spaced from each other so that the region of the master being observed at any instant did not have to be precisely defined.
In a facsimile system, small, side-by-side pixels, each comprising a range of colors, must be observed, and as many as three colors from each pixel must be separated such that the relative intensities of the colors found in the image pixel are preserved in the final electrical output. To utilize a light guide system such as that disclosed by Bryce in a facsimile system, the light guide would have to provide, in each leg, a representative sample of the light intensity observed by the combined light guide across the entire pixel.
An object of the present invention is to provide an optical scanner for obtaining accurate, multicolor signals from each well defined pixel of an image. A further object of this invention is to provide such a scanner which may be produced at a substantially lesser cost than the three color prisms conventionally used.